In data transmission, especially at high bitrate, data decision is a crucial aspect, i.e., to decide for each bit of a received transmission signal on whether the bit content is logical 1 or logical 0. The reason for this being that due to linear and non-linear effects on the transmission path, the received signal is distorted and needs to be restored to an ideal binary signal with steep rising edges. This is especially true for optical binary signals which are subject to dispersion on the transmission path and which need to be converted from optical to electrical and then electrically restored.
Typically, data decision is performed with a decider which compares the signal level of each successive bit with a threshold value. The bit contains logical 1 if the signal level exceeds the threshold and logical 0 otherwise. A problem is that data decision near the threshold value may be defective.
An improved decider circuit known from EP 0 923 204 contains in addition a pseudo-error monitor which controls the threshold value of the decider.
Another decider circuit known from EP 0 912 020 contains three parallel deciders coupled to a multiplexer. The output of the multiplexer is fed to several delay elements which couple back to the multiplexer. This decider circuit serves to compensate for differences in propagation time delay of different signal constituents at different polarization modes of a received optical signal due to polarization mode dispersion.
A special receiver for four-level optical signals is known from EP 0 656 700. A four-level optical signal contains data symbols at different amplitude levels, each symbol representing two data bits. The O/E converted signal is fed in parallel to three deciders each having a different threshold value. The output of the three deciders is fed to a multiplexer which translates the four possible states into the two bits contained therein. The decider and multiplexer thus serves to convert the multi-level signal back into a binary signal. The bitrate of the converted binary signal is twice the symbol rate of the received optical signal. This receiver and corresponding method has also been reported at the ECOC 1996 (see B. Wedding et al, “Multi-Level Dispersion Supported Transmission at 20 Gbit/s over 46 km Installed Standard Single Mode Fiber”, Proceedings of ECOC 1996, Oslo, MoB.4.4, pp.1.91-1.94).
In transmission systems at high bitrate of 10 Gbit/s or even 40 Gbit/s, it is crucial to increase the sensitivity of optical receivers. This is especially true for submarine transmission systems or for transmission systems with a reduced number of regenerators on the signal path.